Thanks for sending along the additional information. The situation you are describing is very strange, especially the ability to log in using the "ftp" credentials, but not the "m1234567" users.
If it were webmail we were talking about, I could understand it, as several of my email accounts have the email email@example.com set up where "address" is the ftp user, and the passwords are the same, but I've never encountered being able to login via POP3/IMAP client with that user name.
I can also identify with the "stuck" "change pending status; that has happened to me before and it is very frustrating. I have no idea why certain DH mail related changes take so long sometimes, and are almost instantaneous other times.
Unfortunately, I have no reasonable explanation as to why the account acts in the way you describe, so at this point I can only sympathize with you (though I'd far rather be able to provide an answer).
In spite of your understandable frustration, I suggest you don't just "give up" on the support requests - If (and when) the "change pending" status goes away, if the problem persists I think you should send yet another support request. The next support tech may do a better job of getting to the bottom of it.